Plutonium is normally recovered from spent nuclear reactor fuel by dissolving the fuel in aqueous nitric acid and then extracting plutonium from the acid solution. A serious difficulty encountered in this process results from the hydrolysis and subsequent polymerization of tetravalent plutonium when the nitric acid fuel dissolving solution is diluted. The thus-formed colloidal plutonium, which will be designated hereinafter as Pu(IV) polymer, is inextractable from the acid solution by the commonly employed solvent extraction and ion exchange processes and tends to precipitate at various stages in the fuel reclamation system, which not only results in loss of plutonium in waste streams but also can plug process equipment and cause a criticality problem.
The formation of Pu(IV) polymer and the nuclear fuel processing problems which it causes have been described in the following publications:
1. "The Preparation and Properties of Some Plutonium Compounds, Part V. Colloidal Quadrivalent Plutonium", published in J. Chem. Soc., p. 3358, in 1956, by D. W. Ockenden and G. A. Welch. PA1 2. "Hazards and Experimental Procedure Evaluation For: Studies on the Polymerization and Hydrolysis of Plutonium in Uranyl Nitrate and Nitric Acid Solutions at Elevated Temperatures", published in 1963 by R. E. Biggers and D. A. Costanzo of the Oak Ridge National Laboratory. PA1 3. "A Study of the Polymerization, Depolymerization, and Precipitation of Tetravalent Plutonium As Functions of Temperature and Acidity by Spectrophotometric Methods: Preliminary Report", published in 1963 by R. E. Biggers and D. A. Costanzo of the Oak Ridge National Laboratory. PA1 4. Volume 1, section 13-2.2, "Plutonium Handbook, A Guide to the Technology", published in 1967 by Gordon and Breach, Science Publishers.
It was disclosed in the publication listed as Item 3 above that Pu(IV) polymer can be depolymerized to ionic plutonium by treating the polymer with 5 molar nitric acid at temperatures in the range of 25.degree.-95.degree. C. Eliminating Pu(IV) by such treatment requires the addition of a large amount of acid to the low-acid nuclear fuel reclaiming streams in which Pu(IV) polymer is formed, which is undesirable.
The publication listed above as Item 4 states that strong complex-forming agents such as fluoride and sulfate ions promote depolymerization of Pu(IV) polymer. However, it will be readily understood by persons skilled in the art of nuclear fuel reclamation that it is undesirable to add fluoride or sulfate ions to process streams, which must be recycled and cannot be adulterated. Fluoride ions would also cause severe corrosion in the process equipment that is generally used in nuclear fuel reclamation.